1. Field of the Invention
This invention relates to method and means for storing digital data. More specifically, it relates to the storage of digital data in a partitioned direct access storage device.
2. Description of the Prior Art
A significant problem in the storage of digital data on direct access storage devices is characterized as "device bottleneck."That is, when a storage device receives access requests from a processor at a faster rate than the device can sustain, the processor is forced to wait. This results, in many cases, in a catastrophic degradation of throughput: the rate at which data records are stored on or retrieved from data sets on the device.
One strategy implemented at some computer installations for managing device bottleneck is to have a person, usually a data base administrator, "manually" examine the device (that is, scan the contents of the device), and select for removal to other devices sufficient data sets so that accesses to the data sets remaining on the device will not exceed the device capacity.
Another strategy for managing device bottleneck implemented by some application programs is to allocate to a data set much more physical space on a device than is required. The space allocated in excess of requirements is, therefore, not written with data by this or other programs, with a resulting decrease in demand on the device from other applications which would, except for the excess allocation, be accessing additional data on the device.
It is recognized, additionally, that by placing data on adjacent tracks of a direct access storage device a faster rate of access can be achieved due to the shorter seek times required. Thus, a data base administrator may allocate physical space on a device to a given data set or group of data sets at one time. However, this strategy for increasing the access rate to data is only effective if there is either no or little competition for access to data residing in other areas of the device.
When a plurality of data storage devices are available, having different access characteristics (such as disk and tape storage devices), it is known to allocate frequently used data to the disks, which are faster, and less frequently used data to the tapes, which are slower. A further approach to the problem of bottlenecks provides an interface unit, including buffer storage and a control processor, which queues data for transfer with respect to a central processor and a plurality of storage devices of varying characteristics.
Generally, however, the storage of data is defined by the user of the system. He prescribes where the data will be placed. Unable to predict the use of data with sufficient precision, the user frequently places data in such a way that the storage device is unable to provide the required number of accesses during periods of peak demand by the central processor, resulting in excessive response times and overall system degradation. This results in considerable effort on the part of the user to tune his system or, in other words, to rearrange his stored data to relieve the excessive access burden on a particular device or set of devices. In the case where the user has several different kinds of storage devices that have different access rates and/or storage capacities, he will attempt, on an intuitive basis, to place the data sets on devices that more nearly provide the performance required, and when using devices that are not well matched to this requirement, he will over allocate space in an attempt to assure a sufficient access rate to a particular data set. Thus, even while not effectively accomplished, a great deal of effort by highly skilled personnel is required to place data sets and to monitor system performance. If data storage capacity is not used effectively (resulting in wasted storage capacity and/or access capability), the data storage system generally operates in a degraded mode with human intervention occurring when degradation becomes intolerable, and data sets frequently are not placed on devices that most nearly meet the access and storage characteristics of the data.
Further, storage devices, which have a relatively narrow range of capability, are being called upon to meet a wide range of demands. In the case of direct access storage devices, there is little freedom to modify the storage capacity or to vary the number of accesses that can be sustained in a given period of time. This results in the storage of data in such a way that the access rates that are required cannot be met or the storage capacity that is available cannot be used if the data to be stored requires a sufficiently different rate of access per unit of capacity than the device is able to provide. In one case, for example, if the accesses per unit of time for each unit of capacity of the storage device exceeds the access rate for each unit of capacity of the data set to be stored the access capability of the device will be under utilized. In the other case, where the access rate per unit volume of data is greater than the access rate per unit volume of storage capacity of the storage device that will provide storage for that data, the device will either be unable to provide the accesses required or the storage capacity cannot be fully utilized. In either case, the cost effectiveness of the storage device is reduced and in the case where sufficient accesses cannot be provided, there is a degradation of the data processing system in terms of throughput and response time.